The invention relates to halogen-free, flame-retardant polymer foams based on styrene polymers, to a process for producing halogen-free, flame-retardant polymer foams, and also to the use of these in the construction industry.
Provision of flame retardants to foams is important for a wide variety of applications, examples being molded polystyrene foams made of expandable polystyrene (EPS) and extruded polystyrene foam sheets (XPS) for the insulation of buildings.
The flame retardants currently used in plastics are mainly polyhalogenated hydrocarbons, if appropriate in combination with suitable synergists, for example organic peroxides or nitrogen-containing compounds. A typical representative of these traditional flame retardants is hexabromocyclododecane (HBCD), which is used by way of example in polystyrene. The plastics industry is making great efforts to find replacements for halogenated flame retardants, because of bioaccumulation, and also because some polyhalogenated hydrocarbons are persistent materials.
Flame retardants should ideally exhibit not only a high level of flame-retardant action in the plastic at a low level of loading but also adequate resistance to heat and hydrolysis for processing purposes. They should also exhibit an absence of bioaccumulation and persistency.
DE 1 694 945 describes a process for producing foams, where the foam incorporates sulfur as flame retardant, alone or in combination with brominated phosphorus compounds.
EP 0 806 451 describes flame-retardant styrene polymer compositions which comprise a combination of organic phosphorus compounds and elemental sulfur. The loading required here in order to achieve satisfactory flame retardancy is mostly at least 10 parts by weight of phosphorus compound and sulfur, based on 100 parts by weight of the polymer.
WO 99/10429 likewise describes flame-retardant styrene polymer compositions which comprise a combination of organic phosphorus compounds and elemental sulfur. The total amounts needed here in order to achieve satisfactory flame retardancy are likewise at least 10 parts by weight of phosphorus compound and sulfur, based on 100 parts by weight of polymer.
The flame-retardant polymer compositions described, in the prior art exhibit satisfactory flame-retardant properties. However, the markedly higher amounts of flame retardants used in the prior art for thermoplastic polymers such as polystyrene can disrupt the foaming process for polymer foams, or can have an adverse effect on the mechanical and thermal properties of the foam.
When expandable polystyrene is produced by suspension polymerization, large amounts of flame retardant can moreover reduce the stability of the suspension. Furthermore, the action of flame retardants used in thermoplastic polymers is often not predictable in polymer foams, because of different fire behavior and different fire tests.
There is therefore major scope for improvement in the foamability of polymer compositions of this type. There is moreover a need for improvement in respect of the flame retardancy and the mechanical properties of halogen-free, flame-retardant polymer compositions and polymer foams.